Assurances that the computers holding the files were disconnected from any networks, and had never been connected to the internet, were not enough to ease GCHQ concerns that the discussions around them could be heard.

How practical is that, though? During the meetings leading up to the destruction, one intelligence agency expert said that if there was a plastic cup in the room, a laser trained on it would be able to pick up the vibrations caused by conversation, and so eavesdrop on them. Or a laser (using non-visible light) could be bounced off a window of the room.

Aficionados of spy films may nod appreciatively: the idea of "laser spying" is a well-known one. It has actually been used by the US against Russian embassies, and has existed as an idea since the 1940s – predating the laser, which was only developed in 1960. A high-quality laser can fire a beam of invisible light for up to half a mile without spreading.

It has even been reported that the CIA used a "laser microphone" to determine that a building in Abbottabad contained a previously unseen male inhabitant – eventually determined to be Osama bin Laden. John Pike, director of GlobalSecurity.org, told NPR in May 2011: "If you shine a laser beam on those windows [of the buildings], you can detect those vibrations, and using voice identification, you can figure out how many different voices are speaking in each of the rooms of the compound."

That wouldn not necessarily yield what was actually being spoken, though. And in normal life such spying probably poses less of a risk than almost any other method of surveillance. "If you want to listen to what's going on in a room, there are far easier ways to do it," Lee Marks, a director at surveillance equipment supplier Spymaster, in London's Portman Square, said. Laser spying, he said, is "about the most difficult way of listening to what's going on in a room that you can come up with".

The principle of laser spying is comparatively simple. The conversation inside a room moves the air; the air moves the windows. A laser beam aimed at the window will shift slightly in wavelength as the window moves. By tracking that shift, the movement of the window can be inferred – yielding the original conversation.

There are audio files online claiming to show the quality of sound that can be achieved – which certainly appear convincing. The problem is aligning the laser, says Marks: "you have to get it exactly at right angles. It has to bounce off and right back to you."

He says his business – which offers a wide range of surveillance and counter-surveillance equipment – has never sold a single laser spying system, which would have to be ordered from a supplier. (An internet search provides plenty of examples.)

That does not mean that it hasn't been used; the US government certainly used it against the Russians. More recently, Nasa technology has been suggested as a method of extending it: in 2005, New Scientist reported that the US security services were using a space technology previously used to detect faint radio signals from space to eavesdrop on a room where the curtains had been pulled (which defeats the laser).

The system used a "horn antenna" to blast a beam of microwave energy at between 30GHz and 100GHz through a building wall: "If people are speaking inside the room, any flimsy surface, such as clothing, will be vibrating. This modulates the radio beam reflected from the surface," it reported. That could be amplified using the Nasa technology and analysed as before.

But is it practical? Only GCHQ knows for sure. At Spymaster, Marks says that he gets "a couple of inquiries a year" from people who want to use the system – but that they are put off when he explains the cost (which starts at about £25,000 for the laser, interferometer, and phototransistor amplifier used to convert the light into sound) and the complexity of carrying it out.